[Patent Document 1] JP 2005-289175 A
There is proposed a vehicle presence notification apparatus which reports or announces a presence of a vehicle using a notice sound (for example, refer to Patent document 1). The vehicle presence notification apparatus in Patent document 1 generates a notice sound using a dynamic speaker that directly generates an audible sound. The vehicle presence notification apparatus is required to generate a notice sound of a predetermined sound pressure (for example, 50 dB) in a predetermined range (for example, the range of 10 m) in the vehicle heading direction (e.g., in front of the vehicle) in order to notify a pedestrian of the approach of the vehicle.
The dynamic speaker generates an audible sound directly; thus, it has a wide directionality (i.e., weak directionality). When generating a notice sound in the dynamic speaker, a loud notice sound sounds also in a direction different from the heading direction of the vehicle, thus providing a cause of vehicle noises.
To address the above, a vehicle presence notification apparatus may use a parametric speaker which has a narrow directionality (i.e., strong directionality) and generates a notice sound at a position distant from the vehicle. The parametric speaker applies an ultrasonic modulation to a “waveform signal of an audible sound (notice sound),” and emits an ultrasonic wave as a modulated signal from an ultrasonic wave speaker. The ultrasonic wave (sonic wave not heard by an ear of a human being) radiated from the ultrasonic wave speaker contains modulated components, which will undergo a self-demodulation in the air in a course of the transmission or propagation. An audible sound (i.e., notice sound) is thereby generated (i.e., become audible) at a position distant from the vehicle.
As mentioned above, the parametric speaker has a narrow directionality and can generate a notice sound only in a limited direction such as a heading direction of a vehicle. When using the parametric speaker, a notice sound may be therefore not generated in a direction different from the heading direction of the vehicle. It is impossible to notify a person in the direction different from the heading direction (e.g., a position in the vehicle-width direction) of the presence of the vehicle.
That is, when only using the dynamic speaker, a notice sound is generated to be too loud in a direction different from the heading direction of the vehicle, causing a disadvantage. When only using the parametric speaker, a notice sound is generated to be too soft in a direction different from the heading direction of the vehicle, causing a disadvantage.